1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing a deterioration of picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):τr∝γd2/Δ∈|Va2−VF2|  (1)where τr represents a rising time when a voltage is applied to a liquid crystal; Δ∈ is a dielectric anisotropy; Va is an applied voltage; VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform a tilt motion; d is a cell gap of liquid crystal cells; and γ is a rotational viscosity of the liquid crystal molecules.τf∝γd2/K  (2)wherein τf represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an elastic constant.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a moving picture is displayed with a brightness lower than the corresponding value of video data VD as in FIG. 1.
Referring to FIG. 1, the conventional LCD cannot express a desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data by using a look-up table (hereinafter referred to as high-speed driving method). This high-speed driving method allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving method modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. This high-speed driving method modulates |Va2−VF2| from the above equation (1) on the basis of a differene of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving method compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
In other words, the high-speed driving method compares most significant bit data MSB of the previous frame Fn−1 and the current frame Fn, respectively. If there is any difference between the most significant bit data MSB, it selects a modulated data and modulates as in FIG. 3.
When the most significant bit data MSB is limited to 4 bits, a look-up table in the high-speed driving method is implemented by the following tables:
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
TABLE 2016324864809611212814416017619220822424000324864809611214416019220822424024024024016016486480961121281601922082242402402402403200326480961121281601922082242402402402404800164880961121281601762082242402402402406400164864961121281441761922082242402402408000163248801121281441761922082242402402409600163248649612814416019220822424024024011200163248648011214416017620822424024024012800163248648096128160176192224240240240144001632486480961121441761922082242402401600016324864809611212816019220822424024017600163248648096112128144176208224240240192001632486480961121281441601922242402402080016324848648096112128160176208240240224001632484864809611212814417619222424024000016324848648096112128144176208240
In the above tables, a left column is a data voltage VDn-1 of the previous frame Fn-1 while an uppermost row is a data voltage VDn of the current frame Fn. Table 1 is a look-up table information in which the most significant bits (i.e., 20, 21, 22 and 23) are expressed by a decimal number format. Table 2 is a look-up table information in which weighting values (i.e., 24 25, 26 and 27) of the most significant 4 bits are applied to a 8-bit data.
In order to reduce the volume of the look-up table, only the most significant bit MSB is modulated. A high-speed driving apparatus is implemented in such a way as in FIG. 4.
Referring to FIG. 4, the conventional high-speed driving apparatus includes a frame memory 43 connected to a most significant bit bus line 42, and a look-up table 44 commonly connected to output terminals of the most significant bit bus line 42 and the frame memory 43.
The frame memory 43 stores the most significant bit data MSB for a frame period, and supplies the stored data to the look-up table. The most significant bit data MSB herein is set to most significant 4 bits among a source data RGB Data In having 8 bits.
The look-up table 44 compares the most significant bit data MSB of the current frame Fn inputted from the most significant bit bus line 42 with the most significant bit data MSB of the previous frame Fn-1 inputted from the frame memory 43, to select and output the corresponding modulated data Mdata. The modulated most significant bit data is added with least significant bit data LSB from a least significant bit bus line 41 prior to sending to the liquid crystal display.
The high-speed driving method and device, which modulates only 4 bits of the most significant bit data MSB occupies relatively a small volume since the data width of the frame memory 43 and the look-up table is 4 bits. Thus, the value of the modulated data registered at the look-up table 44 is limited only to the possible value in the 4 bits as shown in the table 1 and table 2.
Consequently, as shown in FIG. 5, it deviates at the gray level part, indicated by arrows, between the gray level of the data inputted in real and the gray level of the modulated data. As a result, the brightness is changed as much as the deviated portions. In other words, the modulated data should be set more than 4 bits to implement a natural looking moving picture. Nonetheless, since the data width of the look-up table is limited to 4 bits, the modulated data is set to less than 4 bits. As a result, a brightness difference becomes even bigger when a difference between the real gray levels is small.